Neuroendocrine tumors (NETs) can be diagnosed by somatostatin receptor scintigraphy. A somatostatin analog is radiolabeled with a tracer and accumulates in tissues with a high density of somatostatin receptors. The radiation exposure of this examination is similar to that of a computed tomography scan of the abdomen.
What is somatostatin receptor scintigraphy?
Somatostatin receptor scintigraphy is a nuclear medicine imaging technique used primarily to diagnose neuroendocrine tumors (NETs). E.g., in the pancreas. Somatostatin receptor scintigraphy is a nuclear medicine imaging technique used primarily to diagnose neuroendocrine tumors (NET). These express somatostatin receptors in high density, to which octreotide, a synthetic somatostatin analog, binds. This is radioactively labeled, and the gamma radiation emitted is detected by a gamma camera. This allows localization of these tumors, which are often inaccessible to other imaging methods. The method has high sensitivity in the diagnosis of neuroendocrine tumors, with the exception of insulinoma.
Function, effect, and goals
The main application of somatostatin receptor scintigraphy is the diagnosis of neuroendocrine tumors (NET). These are epithelial neoplasms that occur primarily in the abdomen and pancreas. They can be benign or malignant and have an incidence of 1-2 per 100,000 per year. These tumors express somatostatin receptors in high density, which is exploited for nuclear medicine detection. Insulinoma, a tumor arising from the endocrine beta cells (islets of Langerhans) of the pancreas, is the only neuroendocrine tumor that cannot be diagnosed with somatostatin receptor scintigraphy because it lacks such receptors. The radiopharmaceutical used consists of a somatostatin analog, a potent complexing agent, and a gamma emitter called a tracer. A commonly used somatostatin analog is octreotide, which is why this procedure is also called an octreotide scan. Octreotide is bound to the complexing agent, for example DTPA (diethylenetriaminepentaacetic acid) or DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and radioactively labeled shortly before use. This is done, for example, with 111indium, which emits gamma rays and has a half-life of 2.8 days. The compound with DTPA is called 111Indium pentetreotide. Because of this short half-life, it is necessary to perform the radiolabeling immediately before the examination. The radiopharmaceutical is applied intravenously and is distributed throughout the organism by the bloodstream. The octreotide portion of the molecule binds to somatostatin receptors in the body and accumulates in tissues with high receptor density. These are naturally found in certain brain areas such as the hypothalamus, cortex and brainstem. In addition, various tumors and their metastases express this receptor. Somatostatin receptor scintigraphy is particularly valuable for detecting gastroenteropancreatic neuroendocrine tumors (GEP-NET), which are difficult to visualize with other imaging modalities. The octreotide scan shows a very high sensitivity in this case. It is used for primary diagnosis as well as for staging (the determination of the tumor stage) and postoperative control. Furthermore, somatostatin receptor scintigraphy is used for the diagnosis of medullary thyroid carcinomas and Merkel cell tumors and for the differential diagnosis of meningiomas versus neurinomas. Some breast and colon carcinomas also express somatostatin receptors. However, the sensitivity of the octreotide scan is much lower in these cases, so it is not used for the diagnosis of these diseases. Four hours after administration of the radiopharmaceutical, the first gamma camera image is taken. The radioactive isotope is now bound to the somatostatin receptors of the organism via the octreotide moiety and emits gamma radiation as it decays. In areas with a high somatostatin receptor density, increased gamma radiation is produced, which is detected by the gamma camera and displayed as an image. In this way, a tumor can be localized. The examination takes about one hour. It is repeated the next day. The radiopharmaceutical is excreted via the kidneys and intestines.Alternatives to 111indium pentetreotide are, for example, 99technetium tektrotide, with which an even higher sensitivity can be achieved. Other isotopes that can be used are iodine and gallium. The latter is used for positron emission tomography (PET) scans.
Risks, side effects, and hazards
Gamma rays, like X-rays for example, are a type of ionizing radiation. These have the ability to remove electrons from atoms, that is, to ionize them. When molecules of the genetic material, i.e. DNA, are affected, mutations can occur that can cause cancer. Such mutations and molecular changes occur repeatedly in cells due to various causes. In most cases, however, they can be eliminated by the cellular repair systems. In the embryonic phase, however, the organism is particularly sensitive to damaging effects. The consequence of radiation exposure in utero increases the risk of developing cancer in childhood. For this reason, nuclear medical examinations are contraindicated in pregnant women. Every patient should avoid intensive contact with pregnant women and small children on the day of the examination. For children, a strict indication is made and the dose of radiopharmaceutical is reduced according to the age and weight of the child. Since radiopharmaceuticals can accumulate in breast milk, breastfeeding women are advised to possibly pump milk before the examination and to interrupt breastfeeding for a few days after the scintigraphy. The short half-life of the isotopes used in nuclear medicine examinations ensures that the radiation does not remain in the organism for long. The radiation exposure of an octreotide scan in an adult is 13-26 mSv (millisievert). This is roughly equivalent to the radiation exposure of a computed tomography scan of the abdomen. For comparison, a simple lung x-ray has 0.02-0.04 mSv. The natural radiation exposure of the environment is 2-3 mSv per year. Direct side effects are not expected, and intolerance reactions to the applied radiopharmaceutical are extremely rare. Patients taking octreotide as a therapeutic agent must discontinue it several days before the examination.
